WOODY VEGETATION ENCROACHMENT: A DRIVER OF HERBACEOUS SPECIES DIVERSITY LOSS IN A COASTAL FEN

Abstract

Early successional wetland habitat is being lost in temperate regions worldwide as a result of changes in disturbance regimes that allow for the establishment and dominance of woody species. This phenomenon is pronounced in fens, which harbor high numbers of special status plant species that require early successional habitat. We investigated the relationship between woody vegetation encroachment and herbaceous species richness within a northern California coastal fen that has been undergoing encroachment by woody vegetation for ca. 80 years. We established 25 transects within the fen and sampled 338 1x1 m plots located at 4-m intervals along transects. At each plot, we recorded the cover of all woody and herbaceous species, as well as litter, thatch, open water, bare soil, and cover of large woody debris. Modeling indicated that woody vegetation cover, height, species richness, litter cover, and distance from the edge of fen were significant predictors of herbaceous species richness. Vegetation sampling indicated that herbaceous species richness declined with increased woody vegetation cover and height and increased litter cover. Canopy closure was found to result in the complete loss of rare plant species and a significant reduction in herbaceous species richness. The results of this study suggest that the re-introduction of disturbance, specifically those that result in the removal of woody vegetation and litter accumulation, is probably essential to maintain herbaceous species diversity and persistence of special status plant species populations in coastal fens.